A mystery is being investigated at the zoo in Sao Paulo, Brazil, where 10 animals have died suddenly.
All the zoo's animals are now under 24-hour surveillance
Three chimpanzees, three tapirs, three camels and an elephant have been lost since 24 January.
Tests so far have shown most of the animals were killed with sodium fluoroacetate, a banned rat poison.
Police have been monitoring activities at the zoo, in an attempt to discover the identity of the person they are calling the "serial animal killer".
There are no suspects yet, reported the Terra news website, but the police inquiry has focused on members of the zoo staff and visitors.
Police say the killer must have been familiar with the zoo's routine, as well as with chemistry and biology.
The manner of poisoning has mystified investigators. Tests on the animals' food and water revealed no trace of the poison.
Fluoroacetate is colourless and odourless, but just one gram is enough to kill an elephant.
"We are deeply saddened by these events," Fatima Valente Roberti, a biologist at the zoo, told Reuters news agency.
"Many of the workers have been with the zoo for several years and spend more
time with the animals than with their own families.
"We cannot fathom what could possibly lead a person to commit such aggression."
Security has been stepped up at the zoo, with hourly visits to animal enclosures.